Computer Security DD2395 - PowerPoint PPT Presentation

Computer Security DD2395 http://www.csc.kth.se/utbildning/kth/kurser/DD2395/dasak10/ Fall 2010 Sonja Buchegger buc@kth.se Lecture 3 User Authentication KTH DD2395 Sonja Buchegger 1

User Authentication KTH DD2395 Sonja Buchegger 2

User Authentication  fundamental security building block - basis of access control & user accountability  is the process of verifying an identity claimed by or for a system entity  has two steps: - identification - specify identifier - verification - bind entity (person) and identifier  distinct from message authentication KTH DD2395 Sonja Buchegger 3

Means of User Authentication  four means of authenticating user's identity  based one something the individual - knows - e.g. password, PIN - possesses - e.g. key, token, smartcard - is (static biometrics) - e.g. fingerprint, retina - does (dynamic biometrics) - e.g. voice, sign  can use alone or combined  all can provide user authentication  all have issues KTH DD2395 Sonja Buchegger 4

Password Authentication  widely used user authentication method - user provides name/login and password - system compares password with that saved for specified login  authenticates ID of user logging and - that the user is authorized to access system - determines the user’s privileges - is used in discretionary access control KTH DD2395 Sonja Buchegger 5

Password Vulnerabilities  offline dictionary attack  specific account attack  popular password attack  password guessing against single user  workstation hijacking  exploiting user mistakes  exploiting multiple password use  electronic monitoring KTH DD2395 Sonja Buchegger 6

Countermeasures  stop unauthorized access to password file  intrusion detection measures  account lockout mechanisms  policies against using common passwords but rather hard to guess passwords  training & enforcement of policies  automatic workstation logout  encrypted network links KTH DD2395 Sonja Buchegger 7

Use of Hashed Passwords KTH DD2395 Sonja Buchegger 8

UNIX Implementation  original scheme - 8 character password form 56-bit key - 12-bit salt used to modify DES encryption into a one-way hash function - 0 value repeatedly encrypted 25 times - output translated to 11 character sequence  now regarded as woefully insecure - e.g. supercomputer, 50 million tests, 80 min  sometimes still used for compatibility KTH DD2395 Sonja Buchegger 9

Improved Implementations  have other, stronger, hash/salt variants  many systems now use MD5 - with 48-bit salt - password length is unlimited - is hashed with 1000 times inner loop - produces 128-bit hash  OpenBSD uses Blowfish block cipher based hash algorithm called Bcrypt - uses 128-bit salt to create 192-bit hash value KTH DD2395 Sonja Buchegger 10

Password Cracking  dictionary attacks - try each word then obvious variants in large dictionary against hash in password file  rainbow table attacks - precompute tables of hash values for all salts - a mammoth table of hash values - e.g. 1.4GB table cracks 99.9% of alphanumeric Windows passwords in 13.8 secs - not feasible if larger salt values used KTH DD2395 Sonja Buchegger 11

Password Choices  users may pick short passwords - e.g. 3% were 3 chars or less, easily guessed - system can reject choices that are too short  users may pick guessable passwords - so crackers use lists of likely passwords - e.g. one study of 14000 encrypted passwords guessed nearly 1/4 of them - would take about 1 hour on fastest systems to compute all variants, and only need 1 break! KTH DD2395 Sonja Buchegger 12

Password File Access Control  can block offline guessing attacks by denying access to encrypted passwords - make available only to privileged users - often using a separate shadow password file  still have vulnerabilities - exploit O/S bug - accident with permissions making it readable - users with same password on other systems - access from unprotected backup media - sniff passwords in unprotected network traffic KTH DD2395 Sonja Buchegger 13

Using Better Passwords  clearly have problems with passwords  goal to eliminate guessable passwords  whilst still easy for user to remember  techniques: - user education - computer-generated passwords - reactive password checking - proactive password checking KTH DD2395 Sonja Buchegger 14

Proactive Password Checking  rule enforcement plus user advice, e.g. - 8+ chars, upper/lower/numeric/punctuation - may not suffice  password cracker - time and space issues  Markov Model - generates guessable passwords - hence reject any password it might generate  Bloom Filter - use to build table based on dictionary using hashes - check desired password against this table KTH DD2395 Sonja Buchegger 15

Token Authentication  object user possesses to authenticate, e.g. - embossed card - magnetic stripe card - memory card - smartcard KTH DD2395 Sonja Buchegger 16

Memory Card  store but do not process data  magnetic stripe card, e.g. bank card  electronic memory card  used alone for physical access  with password/PIN for computer use  drawbacks of memory cards include: - need special reader - loss of token issues - user dissatisfaction KTH DD2395 Sonja Buchegger 17

Smartcard  credit-card like  has own processor, memory, I/O ports - wired or wireless access by reader - may have crypto co-processor - ROM, EEPROM, RAM memory  executes protocol to authenticate with reader/ computer  also have USB dongles KTH DD2395 Sonja Buchegger 18

Biometric Authentication  authenticate user based on one of their physical characteristics KTH DD2395 Sonja Buchegger 19

Operation of a Biometric System KTH DD2395 Sonja Buchegger 20

Biometric Accuracy  never get identical templates  problems of false match / false non-match KTH DD2395 Sonja Buchegger 21

Biometric Accuracy  can plot characteristic curve  pick threshold balancing error rates KTH DD2395 Sonja Buchegger 22

Remote User Authentication  authentication over network more complex - problems of eavesdropping, replay  generally use challenge-response - user sends identity - host responds with random number - user computes f(r,h(P)) and sends back - host compares value from user with own computed value, if match user authenticated  protects against a number of attacks KTH DD2395 Sonja Buchegger 23

Authentication Security Issues  client attacks  host attacks  eavesdropping  replay  trojan horse  denial-of-service KTH DD2395 Sonja Buchegger 24

Practical Application KTH DD2395 Sonja Buchegger 25

Case Study: ATM Security KTH DD2395 Sonja Buchegger 26

Summary  introduced user authentication - using passwords - using tokens - using biometrics  remote user authentication issues  example application and case study KTH DD2395 Sonja Buchegger 27